From the Microsoft Office 365 Admin center, guest access must be enabled on the Office 365 Groups service. To modify this setting, follow these steps:

1. Log in to the Microsoft 365 Admin Center (https://admin.microsoft.com) with an identity that has been granted the Global Admin role.

2. Navigate to Settings | Services & add-ins | Office 365 Groups:

3. Move the Let group members outside the organization access group content slider to On. Optionally, move the Let group owners add people outside the organization to groups slider to On:

4. Click Save.